Pump control for low flow volumes

ABSTRACT

A method for controlling a gas flow of a pump for high flow rates at a low average flow rate by changing the gas pressure inside a cavity in said pump. The method includes decreasing the gas pressure in said cavity during a first predetermined time period, increasing the gas pressure in said cavity during a second predetermined time period, and stopping the active change of gas pressure during at least a third predetermined time period. Additionally, a pump assembly for high flow rates operated at a low average flow rate is disclosed that includes a number of pumps, a pump motor with a number of stator windings adapted to drive said pumps, and a control unit adapted to control said pump motor. In one embodiment the number of pumps is equal to said number of stator windings. The motor can momentarily increase its force on the pumps.

TECHNICAL FIELD

The present invention relates generally to a method for controlling a gas flow of a pump for high flow rates at a low average flow rate. More particularly, the present invention relates to a method as defined in the introductory parts of claim 1 and to a pump assembly for performing such a method as defined as defined in the introductory parts of claim 14.

BACKGROUND ART

In air sampling scenarios for different pollutants and different analysis methods, the desired airflow rate through different samplers varies much. Certain samplers has their optimal air flow rate at only 1% of the optimal air flow rate for other samplers. It is both expensive and inconvenient to provide one air sampling pump for each air sampling scenario, therefore there is a need for an air sampling pump that can operate at a large range in flow rate, typically down to 1% of its maximum flow rate.

Existing systems that has the possibility to operate at a wide range has a mechanical valve in the flow system, which connects the airflow stream through the air sampler in parallel with an extra inlet, forcing the pump to pull air through both the extra inlet and the air sampler. Thus, the air flow through the sampler are lowered. There are several drawbacks with this method. Firstly, the mechanical valve either requires a manual engagement performed by the user of the instrument or an automated solution inside the instrument operating the valve. Secondly, the ratio of the flow through the air sampler is determined of the flow resistances (restrictions) through the extra inlet and said air sampler. A too high restriction of said extra inlet would make most of the flow still pass through the air sampler, while a too low restriction would make no flow pass through the air sampler if the restriction of the air sampler is too high. Thirdly, this solution is not power efficient since the pump also pulls air through the extra inlet.

There is thus a need for an improved air sampling pump that is easier to operate, may be controlled with higher accuracy and precision avoiding bypass flow channels, and may be automatically operated by an automated system without expensive valve operating servo motors or the like.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the current state of the art, to solve the above problems, and to provide an improved method for controlling a gas flow of a pump so that it may be operated from 1% to 100% of its capacity. These and other objects are achieved by a method for controlling a gas flow of a pump for high flow rates at a low average flow rate by changing the gas pressure inside a cavity in said pump, said method comprising the steps of: decreasing the gas pressure in said cavity, during a first predetermined time period; increasing the gas pressure in said cavity during a second predetermined time period; stopping the active change of gas pressure during at least a third predetermined time period; and wherein the decreased gas pressure and the increased gas pressure are predefined to overcome an opening resistance of an inlet valve and an outlet valve, respectively. When performing theses steps the pump that is capable of a high flow rate may be operated to pump with a very small flow rate. The pump is operated at a fairly high momentary flow rate, but for a short time period. It is then paused for a while. The average flow rate may thus be low while the momentary flow rate is high enough so that the inlet valve and outlet valve for the pump chamber or pump chambers are able to open, since the opening resistance of the inlet valve and outlet valve is overcome. The average flow rate may be controlled by the duration of the first, second, and third predetermined time periods.

For most practical applications it is preferred that the first predetermined time period is substantially equal to said second predetermined time period. The intake and outtake periods for most pumps are operated at equal flow rates, but theoretically, they could be different from each other.

It is further preferred that said first predetermined time period and said second predetermined time period correspond to a part of a pump cycle of said pump. The first predetermined time period, where the pressure inside said pump cavity or pump chamber is decreased, i.e. when the intake valve is forced to open and let gas into the pump cavity, may e.g. be half a pump cycle. The second predetermined time period is then preferably the other half of the pump cycle. For a membrane pump that would be one complete pump cycle, most often corresponding to one revolution of the driving axis for the pump, driven by a pump motor. In case of a rotary pump, as a rotary vane pump, the first and second time periods would be simultaneous.

According to further aspect of the present invention the pump is connected to a control unit, said control unit controlling said gas pressure decrease and said gas pressure increase; and said first predetermined time period and said second predetermined time period. The control unit is in digital or electrical communication with a control program or operator control devices so that it may alter said first, second and third time periods and thereby control the average flow rate from the pump.

According to a still further aspect of the present invention the control unit controls two or more membrane pumps, said pumps having individual pump cycles that are maximally phase shifted in relation to each other. Since membrane pumps by design will produce a pulsed flow, the use of several membrane pumps that all are connected to the same output flow channel, will reduce the pulsation when they are maximally phase shifted relative each other. Additional gas expansion chambers may also be added to further reduce the pulsation. If, e.g. using three membrane pumps, the pump cycles would be phase shifted 120 degrees to each other so as to reduce pulsation as much as possible (one pump cycle being 360 degrees). A further advantage of using multiple membrane pumps in a pump assembly that are phase shifted is that vibrations are reduced prolonging the lifetime of the pump assembly.

It is further preferred that method further comprises the step of: controlling the main gas flow of the pump by repeatedly stopping the active change of gas pressure. This is preferably done by simply stopping the pump motor driving the drive shaft of the pump for the third predetermined time period, the third predetermined time period being set after the desired average flow rate.

The gas pumped is preferably air and the pump may further be adapted to be used in connection with a sampling device for measuring air quality. It is crucial for pumps used for sampling devices to be able to be controlled accurately and to work for the flow rates that are specified for the sampler used.

According to a still further aspect of the present invention the increase of gas pressure and said decrease of gas pressure are controlled by said control unit based on a voltage provided to a pump motor of said pump or pumps. The pump motor may preferably be an electrical motor so that by controlling the voltage of the electrical power driving the motor, the control unit controls the rotational speed of the motor and its drive shaft.

According to a still further aspect of the present invention the voltage provided to said pump motor of said pump or pumps is momentarily substantially higher than the specified input voltage for said pump motor. When pulsing the motor voltage as suggested, the motor can manage a substantially higher input voltage than specified for the motor. The short time duration of the higher voltage means that heat problems do not have time to build up. The momentarily higher input voltage than specified for the motor during the start phase of the pump leads to a more rapid start of the motor compared to a normal start when the motor slowly reaches its operational speed for a certain voltage.

According to a still further aspect of the present invention the method further comprises the steps of detecting a rotational speed of said pump motor using a revolution sensor, e.g. a Hall sensor, measuring an output gas flow from said pump or pumps using a flow sensor, e.g. a differential thermal mass flow meter. The Hall sensor facilitates the possibility to measure an average flow during an integer number of pump cycles, providing a much more reliable calculated average flow rate, especially if the intervals between running pump cycles are long.

According to a still further aspect of the present invention the method further comprises detecting the angle of the rotor in said pump motor; comparing said angle with the positions of said pump or pumps; and providing a higher voltage to the electrical motor to increase angular velocity in order to increase or decrease said gas pressure inside the pump chamber sufficiently fast to let said inlet valve and outlet valve operate. If the control unit detects that the rotational speed of the electrical motor and thus the flow rate through the pump chamber/chambers is lower than what is needed for the inlet and outlet valves to operate properly, i.e. open and close as they should, the voltage to the electrical pump motor may be momentarily increased at the angle of the electrical motor corresponding to the most demanding position of a valve operation.

According to a still further aspect of the present invention the method further comprises to monitoring the output gas flow during one revolution or part of one revolution of the pump motor for detecting possible pump failure of any one of said pump or pumps. If, e.g. using a plurality of membrane pumps, a damaged pump may be detected and identified.

The invention further relates to a pump assembly for high flow rates operated at a low average flow rate comprising: a number of pumps; a pump motor with a number of stator windings adapted to drive said pumps; a control unit adapted to control said pump motor; wherein said number of pumps is equal to said number of stator windings. Since an electrical motor is a lot easier to start at certain angles of the rotor to the stator, it is desired to stop the pump at that position. Using the same number of windings for the pump motor as the number of pumps in the pump assembly, the pump may be pulsed in a manner where one pump finishes a pump cycle, and the motor stops at a position where it is easily started again. The position of the pump may be measured by the Hall sensor to ensure stopping at the correct position, i.e. a position where the pump motor has a maximum torque for starting again. The membrane pump heads are preferably aligned with the correct stopping position so that at least one pump cycle is at its starting position (0 degrees) at that position, i.e. at least one pump head has a full cycle of intake and outlet ahead that is not begun. The stator windings may thus advantageously be positioned relative to said pumps with such an angle that the maximum torque by said rotor and windings is exerted at an angle of said rotor where most force is required to operate said pumps. Many stock driver solutions have a start up phase when starting them in order to find motor position, which start up phase may be avoided using the presented method. This saves power, makes the control of the pump assembly more accurate and makes sure that a start of the pump is as rapid as possible for every time period it is used, no matter how short.

As presented above, the problems of the prior art are thus addressed by the above presented method for controlling a gas flow of a pump for high flow rates at a low average flow rate by running said pump in intervals.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the [element, device, component, means, step, etc.]” are to be interpreted openly as referring to at least one instance of said element, device, component, means, step, etc., unless explicitly stated otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects, as well as additional objects, features and advantages of the present invention, will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic view of a membrane pump with one membrane.

FIG. 2 is a schematic view of a membrane pump having four pump heads, i.e. four membranes.

FIG. 3a illustrates the function of the valves for a membrane pump membrane when the pressure in the pump chamber is decreased during inlet of gas.

FIG. 3b illustrates the function of the valves for a membrane pump membrane when the pressure in the pump chamber is increased during outlet of gas.

FIG. 4 illustrates malfunction identification of a four membrane pump according to the present invention.

FIG 5a is a schematic view of a the pump motor windings of a membrane pump.

FIG 5b is a schematic view of a the pump motor windings of a membrane pump in relation to the three pump heads of the membrane pump.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 is a schematic view of a membrane pump with one membrane, showing only the drive shaft 1, the crank shaft 2 and the membrane 11. The pump chamber and the inlet and outlet valves are not shown. FIG. 2 is a schematic view of a membrane pump of a similar type as in FIG. 1 but having four pump heads, i.e. four membranes 11, 12, 13, 14. The use of four pump heads will reducing the maximum torque required by the pump motor and it will achieve a smother flow since the pump cycles of the four individual pump cycles are phase shifted 90 degrees to each other.

FIG. 3a illustrates the function of the valves 3, 4 for a membrane pump membrane when the pressure in the pump chamber 5 is decreased during inlet of gas. As can be seen the inlet valve 3 is forced to open while the outlet valve 4 is forced to close due to the decrease in pressure in the pump chamber 5 and a flow of gas indicated by the arrow 17 enter through the inlet valve 3 into the pump chamber 5. FIG. 3b illustrates the function of the valves 3, 4 for a membrane pump membrane when the pressure in the pump chamber 5 is increased during outlet of gas. As can be seen the inlet valve 3 is forced to close while the outlet valve 4 is forced to open due to the increase in pressure in the pump chamber 5 and a flow of gas indicated by the arrow 18 exit through the outlet valve 4 exiting the pump chamber 5. When running membrane pump heads with passive valves, a certain speed is required to induce enough backpressure dΔP/dt change for opening valves. Due to the torque required to compress the membrane at each pump head, it is hard for a small electrical motor to drive the pump heads slowly with a fixed angular velocity. By pulsing one revolution at a time, these problems are overcome. A high enough backpressure is induced and the electrical motor can be precisely controlled. High accuracy, high speed and instant (simple) data processing of non-averaged values are required in order to measure flow correctly when induced with such a pump. This is achieved by a revolution sensor, preferably a hall sensor (not shown), keeping track of the pump revolution speed. The flow is measured by a mass flow meter.

Data processing of flow curve and characterization of flow pulses induced by membrane strokes gives a diagnostic indication of pump condition as shown in FIG. 4. FIG. 4 illustrates malfunction identification in a four membrane pump according to the present invention.

For starting the pump according to the present invention when pulsing one revolution at a time, a power boost (e.g. higher voltage on driver bridges) may assist a pump motor otherwise too weak. By measuring how the pump motor manages to keep speed up (depending on load) the signal to the driver bridges can be reduced while maintaining the same pace. Accurate control of the pump motor may thus be achieved.

FIG. 5a shows a schematic image of the motor 20 for the pump assembly according to the invention. The motor has three windings 21, 22, 23 and a dipole magnet 24, 25 having a north pole 24 and a south pole 25, said dipole magnet having the drive shaft 1 of the pump assembly arranged in the center of the magnet. The rotation of the magnet and the drive shaft is indicated by the arrow 26. In FIG. 5a the motor is positioned for an easy start by having the winding 21 pull the north pole 24 towards it, while the winding 22 is operated in counter phase so that it will push the north pole 24. In that way the rotation is initiated. The rotation is then continued in a normal manner for driving an electrical motor, well known for a person skilled in the art.

FIG. 5b shows the schematic image of the motor 20 as in FIG. 5 a, having three pump heads 31, 32, 33 superposed on the engine 20 attached to the drive shaft 1 of the pump assembly. The pump head outlets are connected to a common outlet flow channel (not shown) and the pump head inlets are connected to a common inlet channel (not shown).

It is understood that other variations in the present invention are contemplated and in some instances, some features of the invention can be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly in a manner consistent with the scope of the invention. 

1. A method for controlling a gas flow of a pump for high flow rates at a low average flow rate by changing the gas pressure inside a cavity in said pump, said method comprising the steps of: decreasing the gas pressure in said cavity, during a first predetermined time period; increasing the gas pressure in said cavity during a second predetermined time period; stopping the active change of gas pressure during at least a third predetermined time period; and wherein the decreased gas pressure and the increased gas pressure are predefined to overcome an opening resistance of an inlet valve and an outlet valve, respectively.
 2. The method of claim 1, wherein said first predetermined time period is substantially equal to said second predetermined time period.
 3. The method of claim 1, wherein said first predetermined time period and said second predetermined time period correspond to a part of a pump cycle of said pump.
 4. The method of claim 1, wherein said pump is connected to a control unit, said method including controlling with said control unit said gas pressure decrease and said gas pressure increase; and said first predetermined time period and said second predetermined time period.
 5. The method of claim 1, wherein said pump is a membrane pump.
 6. The method of claim 4, wherein said control unit controls two or more membrane pumps, said pumps having individual pump cycles that are maximally phase shifted in relation to each other.
 7. The method of claim 5, further comprising the step of: controlling the main gas flow of the pump by repeatedly introducing a short interrupt of flow between one or several pump cycles of said membrane pump for reducing the average gas flow over time.
 8. The method of claim 1, wherein said gas is air.
 9. The method of claim 1, wherein said pump is adapted to be used in connection with a sampling device for measuring air quality.
 10. The method of claim 4, wherein said pump includes a pump motor, said pump motor is an electrical motors and said method includes controlling the increase of gas pressure and said decrease of gas pressure by said control unit based on a voltage provided to a pump motor of said pump or pumps.
 11. The method of claim 10, further including momentarily increasing said voltage provided to said pump motor of said pump or pumps to a voltage substantially higher than a specified input voltage for said pump motor.
 12. The method of claim 10, wherein said pump motor includes a rotor, the method further comprising the steps of: detecting an angle of the rotor in said pump motor; comparing said angle with the positions of said pump or pumps; and providing a higher voltage to the electrical motor to increase angular velocity in order to increase or decrease said gas pressure inside the pump chamber sufficiently fast to let said inlet valve and outlet valve operate.
 13. The method of claim 12, further comprising the step of monitoring output gas flow during one revolution or a part of a revolution of the pump motor for detecting possible pump failure of any one of said pump or pumps.
 14. A pump assembly for high flow rates operated at a low average flow rate comprising: a plurality of pumps; a pump motor with a number of stator windings adapted to drive a rotor, said rotor driving the crank shafts of said plurality pumps; a control unit adapted to control said pump motor; wherein said number of pumps is equal to said number of windings.
 15. The pump assembly according to claim 14, wherein said stator windings are positioned relative to said pumps with such an angle that the maximum torque by said rotor and windings is exerted at an angle of said rotor where most force is required to operate said pumps. 